Entry types

When entering a reference in the bibliography database, the first
thing to decide is what type of entry it is. No fixed classification
scheme can be complete, but BibTeX provides enough entry types to
handle almost any reference reasonably well.

References to different types of publications contain different
information; a reference to a journal might include the volume and
number of the journal, which is usually not meaningful for a book.
Therefore, database entries of different types have different fields
for each entry type, the fields are divided into three classes:

Required

omitting the field will produce an error message and may
result in a badly formatted bibliography entry. If the
required information is not meaningful, you are using
the wrong entry type.

Optional

the field's information will be used if present, but can
be omitted without causing any formatting problems. A
reference should contain any available information that
might help the reader, so you should include the optional
field if it is applicable.

Ignored

the field is ignored. BibTeX ignores any field that is
not required or optional, so you can include any fields
you want in a bibliography entry. It's often a good idea
to put all relevant information about a reference in its
bibliography entry - even information that may never
appear in the bibliography. For example, if you want to
keep an abstract of a paper in a computer file, put it in
an 'abstract' field in the paper's bibliography entry.
The bibliography database file is likely to be as good a
place as any for the abstract, and it is possible to
design a bibliography style for printing selected
abstracts.